Method of cataphoretically providing a uniform layer, and colour picture tube comprising such a layer

ABSTRACT

A uniform dark layer of soot is cataphoretically deposited from a suspension of soot in insulating hydrocarbons on an aluminized phosphor screen of a color display tube in which the aluminum layer serves as one electrode and a shadow mask serves as the other electrode and the potential difference between the electrodes is maintained during decanting the suspension.

United States Patent [1 1 Strik METHOD OF CATAPHORETICALLY PROVIDING A UNIFORM LAYER, AND COLOUR PICTURE TUBE COMPRISING SUCH A LAYER Inventor:

Francis Bemardus Strik, Emmasingel, Eindhoven, Netherlands U.S. Philips Corporation, New York, NY.

Filed: Dec. 1, 1971 Appl. No.: 203,769

Assignee:

[30] Foreign Application Priority Data Dec. 12, 1970 Netherlands 7018154 US. Cl..... 117/226, 117/33.5 C, 117/33.5 CM,

' 117/33.5 CP, 204/181 Int. Cl. C090 l/44, B44d l/02 Field of Search 117/226, 33.5 R, 33.5 C,

[451 Apr. 30, 1974 117/33.5 CM, 33.5 CF

[56] References Cited UNITED STATES PATENTS 3,037,923 6/1962 Gnau 204/181 3,360,450 12/1967 Hays 3,674,670 7/1972 Erikson 204/ 181 Primary ExaminerLeon D. Rosdol Assistant Examiner-Michae1 F. Esposito Attorney, Agent, or Firm-Frank R. Trifari [5 7] ABSTRACT A uniform dark layer of soot is cataphoretically deposited from a suspension of soot in insulating hydrocarbons on'an aluminized phosphor screen of a color display tube in which the aluminum layer serves as one electrode and a shadow mask serves as the other electrode and the potential difference between the electrodes is maintained during decanting the suspension.

5 Claims, 3 Drawing Figures Y 'MIENTEWR Q 3808348 IIIIIIILII INVENTOR. FRANClS B. STRIK i wa Lf- MPNT METHOD OF CATAPIIORETICALLY PROVIDING A UNIFORM LAYER, AND COLOUR PICTURE TUBE COMPRISING SUCH A LAYER The invention relates to the cataphoretic provision of a uniform layer on a conductive surface, in particular a uniform dark layer of carbon or manganese dioxide on an aluminized phosphor screen of a colour television picture tube. The invention furthermore relates to a colour picture tube having a phosphor screen which is provided with such a uniform dark layer.

It is known, for example from German Patent Specification 625,217 and US. Pat. No. 3,037,923 that, for example carbon in the form of soot or graphite, can be cataphoretically provided on a conductive article, in which the carbon is dispersed in a non-conductive suspension medium and the carbon particles obtain a positive potential relative to the suspension medium. These known methods give good results if the surfaces to be coated are comparatively small.

In covering large areas, for example larger than a few sq.dm, a layer is often obtained, however, which is not uniform but shows stripes. From investigations it has been established that said stripes are caused in that during the separation of the covered surface and the suspension, parts of the layer can be carried along by the suspension which moves along the surface. This is disadvantageous in particular if the layer is provided on a phosphor screen of a picture tube since in that case the stripes are visible on the picture surface. The provision of adark layer on the cathode side of an aluminized phosphor screen of a colour picture tube actually is often necessary to obtain a more uniform temperature distribution on the colour selection electrode since in that case no thermal energy radiated by said electrode is reflected back to said electrode by the aluminium layer and said electrode can thus better dissipate its thermal energy by radiation.

The above drawback can be avoided entirely so that cataphoretically an entirely uniform layer can be obtained on a large surface upon covering a conductive surface by cataphoresis from a non-conductive suspension in which the surface to be covered forms one electrode and obtains a potential difference relative to another oppositely located electrode if, according to the invention, during separation of the covered surface and the suspension the potential difference necessary for the cataphoresis between the covered surface and the other electrode is maintained. As a result of the electrostatic force exerted by the potential difference, the particles of the layer arekept in their places when the suspension flows along them upon separating the covered surface and the suspension, so for example during drawing of the surface from the suspension or during the removal of the suspension in the case in which the surface is concave and the suspension is present in a cup-shaped article, for example, the window of a cathode ray tube.

In order that the invention may be readily carried into effect, one embodiment thereof will now be described in greater detail, by way of example, with reference to the accompanying drawing, in which FIG. 1 shows diagrammatically a device for carrying out the method according to the invention, while FIG. 2 shows a colour picture tube having a layer obtained according to the invention, and

FIG. 3 shows a detail hereof.

Referring now to FIG. 1, reference numeral 1 denotes a cup-shaped window of a cathode-ray tube for colour television display. The cup-shaped window 1 is secured to a table 2 by means of a sucker 3. The table 2 can pivot about a shaft 4 and be tilted through by means of an arm 5 and a drawing device 6. As shown in FIG. 3, a phosphor layer 7 is present on the inner surface of the window I and is coated with a conductive aluminium layer 8. A shadow mask or another electrode 9 is present in the window and is connected to the side wall of the window I by means of resilient members 10. A suspension 11 of manganese dioxide (MnO or carbon in the form of soot in a nonconductive liquid, for example ethyl alcohol or acetone or a mixture thereof, is poured in the cup-shaped window l. The suspended particles assume a positive potential relative to the liquid. Instead of ethyl alcohol, the hydrocarbon commercially known as Shell-Sol has proved to be very suitable.

The aluminium layer 8 is connected to the negative terminal 17 of a voltage source by means of a clamping spring 12. The other electrode, in this case a shadow mask 9, is connected to the positive terminal 18 of the voltage source by means of a terminal 13. The potential of the voltage source 14 is, for example, 300 V direct voltage when using a suspension of soot in insulating hydrocarbons of, for example, the above-mentioned brand. Higher potential differences are also possible and accelerate the process.

After a few minutes, the table 2 is pivoted about the axis 4 without interrupting the potential difference so that the suspension is decanted from the cup-shaped window into a receptable not shown. The'surface of the electrode 8 is then coated with a layer of soot 16. Since the electrostatic field between the electrode 9 and the aluminium layer 8 is maintained, no soot particles are removed from the surface of the layer 16 by the suspension 11 which is flowing along and a uniform black layer of soot 16 is obtained.

The use of an electrode 9 in the form of a shadow mask. has the advantage that the soot particles of the suspension 11 can enter the space between the mask 9 and the aluminium layer 8 through the apertures of the mask without it being necessary for the suspension to flowbetween said electrodes. As a result of this the layer 16 can also become very uniform while damage to the thin aluminium layer is avoided. However, the mask 9 is preferably not the mask to be mounted ultimately in the tube, but it may be any mask, possibly a rejected one, which can be used many times for the same purpose. 1 Although one embodiment has been described, the invention may also be applied to coating large surfaces with materials other than manganese dioxide or carbon, for example on covering large metal surfaces with, for example, alumina or other insulating or possibly luiaesc Ls b a s What is claimed is:

l. A method of cataphoretically coating an aluminized surface of a cathode ray tube phosphor screen, comprising the steps of:

a. disposing an electrode opposite said aluminized surface;

b. providing between said electrode and said aluminized surface a suspension comprising a material selected from the group consisting of manganese dioxide and carbon in a medium of a liquid hydrotrode is perforated.

3. A method as defined in claim 2, wherein said electrode is a shadow mask.

4. A method as defined in claim 1, wherein said hydrocarbon is at least one of ethyl alcohol and acetone.

5. A cathode-ray tube comprising an aluminized phosphor screen coated with one of the group consisting of carbon and manganese dioxide, said screen being so coated by the method of claim 1. 

2. A method as defined in claim 1, wherein said electrode is perforated.
 3. A method as defined in claim 2, wherein said electrode is a shadow mask.
 4. A method as defined in claim 1, wherein said hydrocarbon is at least one of ethyl alcohol and acetone.
 5. A cathode-ray tube comprising an aluminized phosphor screen coated with one of the group consisting of carbon and manganese dioxide, said screen being so coated by the method of claim
 1. 